Increasing interest is being expressed in magnetic bearings of various sorts In operation, such bearings journal a rotary part by means of a magnetic field. Actual contact between relatively rotating parts is completely avoided so long as the magnetic bearing is performing its function As a consequence, the bearing is virtually friction free and the only losses associated with rotation of a rotary part are so-called windage losses induced by aerodynamic drag on the surface of the rotating part.
Nonetheless, apparatus employing magnetic bearings requires provision for mechanical bearings as well. In the event of a power failure, or at the time of start up or shut down, a magnetic field sufficient to prevent mechanical contact between relatively rotating parts will not exist and mechanical contact will, in fact, result. If provision is not made to minimize friction resulting from such contact, substantial damage to the apparatus can result Consequently, it is the norm to provide some sort of a mechanical back-up bearing for a magnetic bearing to prevent the possibility of damaging frictional contact between relatively moving parts in such instances. Indeed, many rather elaborate schemes have evolved. For example, in U.S. Pat. No. 4,629,261 issued Dec. 16, 1986 to Eiermann, et al., a rotatable collar is journaled by ball bearings to a stator structure and is axially, but not radially, movable. In the event of a power failure or during start up or shut down, the stator structure is urged under the bias of a spring in an axial direction to engage the collar with a part of the rotary element to be journaled such that the collar rotates with the rotary element to provide a journal for the rotary element through the ball bearings to the stator.
While this approach may be effective, implementation is expensive because of the complexity of the same. Furthermore, to incorporate it in an apparatus requires an expansion of the overall size of the apparatus in order to house the back-up bearing and provide space for the requisite axial movement. Unfortunately, the drawbacks of complexity and undesirably large size are commonplace in back-up arrangements for magnetic bearings
The present invention is directed to overcoming one or more of the above problems.